1. Field of the Invention
The present invention relates to a color measuring device, an image capturing device, an image forming apparatus, a color measurement method, and a computer program product.
2. Description of the Related Art
Inkjet-type image forming apparatuses cause a carriage on which a print head is mounted to reciprocate in a main-scanning direction, and cause the print head to eject ink from nozzle arrays during the reciprocation, thereby printing an image (dots) on a recording medium. Such inkjet-type image forming apparatuses convey the recording medium in a sub-scanning direction by using carriage rollers and the like, and repeat recording in the main-scanning direction, thereby forming an image on the recording medium.
In such an inkjet recording device, the ejection amount of ink ejected from the print head changes because of the state of nozzles in the print head, fluctuation in viscosity of the ink, and fluctuation in a piezoelectric element for driving to eject the ink, for example. As a result, fluctuation occurs in color reproduction of an image formed by the ink ejected from the print head. Furthermore, the ejection amount of ink changes over time in a single device, or varies in different devices. As a result, fluctuation occurs in color reproduction of an image formed by the ink ejected from the print head in this case as well.
To address this, for example, some of the inkjet-type image forming apparatuses described above perform control such that color reproduction of an image formed by the ink ejected from the print head is stabilized by: forming a test pattern to be a target for color measurement on a recording medium; measuring the colors of the test pattern by using a colorimeter; and correcting the ejection amount of ink ejected from the print head based on the result of color measurement of the test pattern.
Examples of the colorimeter described above include a colorimeter that performs color measurement by using a spectroscope, and a colorimeter that performs color measurement by receiving reflected light from the test pattern. These spectroscopes can improve the accuracy in color measurement, but are expensive. Providing such spectroscopes to the image forming apparatus leads to an increase in costs. By contrast, while the spectroscope using reflected light is reasonable in price, the reflected light changes depending on environmental conditions. As a result, the accuracy in color measurement deteriorates.
Therefore, it is necessary to develop a device that is reasonable in price and that performs color measurement with high accuracy.
Japanese Patent Application Laid-open No. 2009-239419 discloses a technology for creating a profile that achieves high color reproducibility. In Japanese Patent Application Laid-open No. 2009-239419, a reference color patch is captured by a spectroscope and a camera, a transformation matrix between a device-dependent color space and a device-independent color space is created, and a more detailed conversion table in the color space is created by using the transformation matrix. Subsequently, an error in the reference color patch converted by the transformation matrix is displayed to enable a user to find the error and correct a profile (conversion table) created from the transformation matrix at his or her judgment. Thus, a profile (conversion table) that achieves high color reproducibility is created.
In the technology disclosed in Japanese Patent Application Laid-open No. 2009-239419, the reference color patch is captured by the spectroscope and the camera, whereby the transformation matrix between the device-dependent color space and the device-independent color space is created. Japanese Patent Application Laid-open No. 2009-239419 requires the spectroscope, the color patch, and a standard light source every time an image is captured to create a profile. Providing the spectroscope, which is expensive, to the configuration leads to an increase in costs.
In Japanese Patent Application Laid-open No. 2009-239419, an error in the color patch after the matrix transformation is displayed to enable the user to correct the profile at his or her judgment. As a result, the conversion accuracy varies between individual users.
In Japanese Patent Application Laid-open No. 2009-239419, the transformation matrix is created by the least squares method. As a result, while the overall average value reaches the minimum, a color range with a large error partially remains. Furthermore, to create the transformation matrix by the least squares method, linear algebra calculation (e.g., inverse matrix calculation) needs to be performed. As a result, the amount of calculation becomes enormous.
Therefore, there is a need for a color measuring device that is reasonable in price and that facilitates improvement in the accuracy in color measurement.